A variable frequency drive controls the speed and torque of an alternating current (AC) motor by varying the input frequency and voltage. Three-phase motors provide higher mechanical efficiency, higher power factor and less torque ripple than single-phase motors and are therefore a more desirable choice. Variable frequency drives in the past have generally included a diode rectifier, that converts AC power to direct current (DC) power, connected through a DC bus to an inverter that supplies three phase, variable frequency AC power to a three-phase motor.
When a motor turns faster than the speed designated by the variable frequency drive, the motor acts as a generator, generating power that is returned to the DC bus. In a variable frequency drive with a diode rectifier, the rectification of the AC power to the DC bus is a one-way street and the generated power causes the voltage on the DC bus to rise.
One known method of handling the generated power is to add a dynamic braking resistor to the variable frequency drive. When the voltage on the DC bus rises due to the generated power, the generated power is shunted to the dynamic braking resistor that converts the generated power to heat. Dynamic braking resistors add complexity and expense to a variable frequency drive installation.
The generated power can alternatively be handled with a regenerative variable frequency drive that has an active converter instead of the one-way diode rectifier. An active converter allows power to flow from the AC source to the DC bus and from the DC bus back to the AC source. A regenerative variable frequency drive puts the generated power back onto the line, and thereby reduces the total power consumption of the load.
Regenerative variable frequency drives with three-phase active converters are known. A conventional diode rectifier drive can convert AC power from a single-phase source to charge the DC bus. The known three-phase active converters cannot convert the power from a single-phase AC source to charge the DC bus.
Three-phase AC power is generally supplied to industrial areas. However, only single phase AC power is available to most residential and rural areas. The single phase AC power available in most residential and rural areas is provided by a step down transformer connected to a high voltage line and, in the United States, is normally supplied as about 240 volts at 60 Hz between the first and second input lines. Many three-phase induction motors are operated at high voltage such as about 460 volts to reduce the current passing between the inverter of the variable frequency drive and the motor, and thereby reducing the required size of the connecting cables. Diode rectifier converters cannot directly boost the incoming 240 volts to 460 volts.
Diode rectifiers distort the current drawn from the power grid. This distortion creates harmonic distortions that may affect other users on the grid. The distortion also reduces the power factor. A variable frequency drive with a diode rectifier therefore requires additional circuitry for power factor correction and harmonic filtering.